Composite material and its manufacturing method

Abstract

To provide a composite capable of achieving both of reduction of a heat conductive particle amount and suppression of anisotropism of a heat transfer coefficient.SOLUTION: There is provided a composite formed of a resin, and heat conductive particles having a great heat transfer coefficient. The resin has a granular island-shaped part, and the heat conductive particles are flat or fibrous. The composite has a plurality of small areas where a first orientation degree is different by 30° or greater, when determining a first orientation degree being an average value of an inclination angle with respect to a reference direction of the heat transfer particles which are present in the island-shaped part for each small area of a square-shape whose one side has a length of twice of an average particle diameter of the heat transfer particles, and in addition, a second orientation degree being an average value of the inclination angle to the reference direction of the heat transfer particles which are present in the island-shaped part and being determined in a large area of the square-shape whose one side has a length of five times of the average particle diameter of the heat transfer particle, is 20°or greater. Such a composite is configured so that a ratio (λ1 / λ2, λ1≥λ2) of the heat transfer coefficient measured for each orthogonal direction becomes 3 or smaller, thereby suppressing anisotropism of the heat transfer coefficient.SELECTED DRAWING: Figure 9

Description

[Technical Field]

[0001] The present invention relates to a composite material having excellent thermal conductivity. [Background technology]

[0002] Electronic devices (such as semiconductor modules) that have become increasingly dense and performant require heat dissipation to maintain their functionality and lifespan. Heat dissipation from electronic devices is typically achieved through heat dissipation components (heat sinks, housings, etc.) made of metal or other materials. In this case, a heat dissipation sheet (such as a thermally conductive sheet or a thermally conductive insulating sheet) is often inserted between the electronic device (heat source) and the heat dissipation component to absorb unevenness or undulations on their surfaces.

[0003] The heat dissipation sheet uses, for example, a composite material (including a composition) made of a filler with high thermal conductivity and a resin (including elastomer, rubber, etc.) that has excellent flexibility (elasticity) and adhesion. Various proposals have been made regarding such composite materials, and for example, the following patent documents contain related descriptions. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2016-255055 Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 proposes an isotropic, highly thermally conductive, electrically insulating composition in which spherical diamonds (first filler) and flaky hexagonal boron nitride (second filler) are dispersed in silicone (resin). Although the composition (composite) uses a large amount of expensive diamonds with high thermal conductivity, the thermal conductivity of the entire composition remains at a maximum of several W / mK.

[0006] The present invention has been made in view of the above circumstances, and has an object to provide a new composite material etc. having excellent thermal conductivity properties. [Means for solving the problem]

[0007] As a result of intensive research into solving this problem, the inventors have changed their approach and, instead of dispersing thermally conductive particles (filler) uniformly in the resin (matrix) as in the past, have succeeded in obtaining a composite material in which the thermally conductive particles are unevenly distributed (aggregated, aggregated, etc.) between island-shaped resin portions, while suppressing the anisotropy of thermal conductivity. By expanding on this result, the present invention, which will be described below, has been completed.

[0008] 《Composite material》 (1) The present invention is a composite material comprising a resin and thermally conductive particles having a higher thermal conductivity than the resin, wherein the resin has granular island portions, the thermally conductive particles are flat or fibrous, and when a first orientation degree, which is the average value of the inclination angle of the thermally conductive particles between the island portions relative to a reference direction, is calculated for each small square region having a side twice the average particle size of the thermally conductive particles, the composite material has a plurality of small regions having a first orientation degree that differs by 30° or more.

[0009] (2) The present invention also provides a composite material comprising a resin and thermally conductive particles having a higher thermal conductivity than the resin, the resin having granular island portions, and the thermally conductive particles being flat or fibrous, This composite has a second orientation degree of 20° or more, which is the average value of the inclination angle of the thermal conductive particles between the island-shaped portions relative to a reference direction, measured in a large square area with one side five times the average particle size of the thermal conductive particles.

[0010] (3) Furthermore, the present invention provides a composite material comprising a resin and thermally conductive particles having a higher thermal conductivity than the resin, wherein the resin has granular island portions, the thermally conductive particles are flat or fibrous, and the ratio (λ1 / λ2) of thermal conductivities (λ1≧λ2) measured in two perpendicular directions is 3 or less.

[0011] (4) The thermally conductive particles of the present invention are unevenly distributed (crowded or aggregated) between granular island-shaped portions, and the islands are in close proximity or contact with each other. Because these thermally conductive particles link together to form thermal conduction paths, the composite material of the present invention can exhibit high thermal conductivity. Furthermore, by reducing the amount of thermally conductive particles due to uneven distribution, the thermal conduction paths are efficiently formed, reducing raw material costs and the overall modulus of elasticity (Young's modulus). Therefore, according to the present invention, a composite material with excellent thermal conductivity, flexibility (adhesion), etc. can be obtained at low cost.

[0012] Furthermore, the orientation of the flat or fibrous thermally conductive particles with a large aspect ratio varies moderately among the island-shaped regions, which results in a composite material of the present invention with a small directional difference in thermal conductivity (anisotropy), and excellent ease of handling and versatility.

[0013] <<Manufacturing method of composite materials>> The present invention can also be understood as a method for manufacturing a composite material. For example, the composite material described above can be obtained by a manufacturing method including a molding step of pressure-molding a mixture of a resin raw material and a thermally conductive particle raw material. The resin raw material may include, for example, granular resin that forms at least island-shaped portions. Furthermore, the thermally conductive particle raw material may be, for example, secondary particles formed by agglomeration of at least flat or fibrous thermally conductive particles.

[0014] The mixture may be heated to a temperature equal to or higher than the softening point of the resin raw material before or during the molding step, so that at least a portion of the resin raw material is softened or melted (e.g., in a solid-liquid coexistence state) (heating step). The heated mixture may be cooled while being pressurized during or after the molding step.

[0015] <Heat dissipation components> The present invention can also be understood as a heat dissipation member made of the above-mentioned composite material. The heat dissipation member may be, for example, a bulk composite material processed (by slicing, cutting, machining, etc.) into a specific shape (sheet, block, etc.), or may be a material integrally molded into a desired shape by compression molding, injection molding, etc. The heat dissipation member used in electronic devices, etc. may be a thermally conductive insulating material.

[0016] "others" (1) In this specification, the term "material" means "material" or "component." Composite materials include composite materials with indefinite shapes (raw materials, intermediate materials, etc.) as well as composite members that have been molded or processed into specific shapes.

[0017] As used herein, "resin" (including resin raw materials) includes elastomers, rubbers, etc. Such resins may be thermoplastic or thermosetting.

[0018] (2) Unless otherwise specified, "x to y" in this specification includes a lower limit value x and an upper limit value y. Any numerical value included in the various numerical values ​​or numerical ranges described in this specification may be used as a new lower limit or upper limit value to create a new range such as "a to b." Unless otherwise specified, "x to y μm" in this specification means x μm to y μm. The same applies to other unit systems (MPa, W / mK, etc.). [Brief explanation of the drawings]

[0019] [Figure 1A] 1 is an SEM image of the composite material of Sample 1. [Figure 1B] 1 is an SEM image of the composite material of Sample 2. [Figure 1C] 1 is an SEM image of the composite material of Sample 3. [Figure 2] This is an SEM image of the composite material used to calculate the aspect ratio of the thermally conductive particles. [Figure 3] 1 is a bar graph comparing the thermal conductivity of an unevenly distributed sample and a uniform sample. [Figure 4A] 1 is an SEM image of the composite material of Sample 4. [Figure 4B] 10 is a bar graph showing the thermal conductivity of Sample 4. [Figure 5] FIG. 1 is a schematic diagram showing the contact or intimate contact state of fillers (thermal conductive particles) in a matrix (resin). [Figure 6] FIG. 1 is a schematic diagram showing the change in filler (thermal conductive particle) spacing depending on whether or not an island portion is present. [Figure 7]These are SEM images of secondary and primary particles of BN particles used as the thermally conductive particle raw material. [Figure 8] These are SEM images of the cross section of a composite material made using these materials. [Figure 9] 1 is a bar graph showing the anisotropy of thermal conductivity of composite materials. [Figure 10A] 10 is a diagram showing the process of calculating the degree of primary orientation based on an SEM image of sample 21. FIG. [Figure 10B] 10 is a diagram showing the process of calculating the degree of primary orientation based on an SEM image of sample 22. FIG. [Figure 10C] FIG. 10 is a diagram showing the process of calculating the degree of primary orientation based on an SEM image of sample C1. [Figure 11A] 10 is a diagram showing the calculation process of the first orientation degree and the second orientation degree based on the SEM image of sample 21. FIG. [Figure 11B] 10 is a diagram showing the process of calculating the first orientation degree and the second orientation degree based on an SEM image of sample 22. FIG. [Figure 11C] FIG. 10 is a diagram showing the calculation process of the first orientation degree and the second orientation degree based on the SEM image of sample C1. DETAILED DESCRIPTION OF THE INVENTION

[0020] One or more components arbitrarily selected from this specification may be added to the components of the present invention. The contents described in this specification apply not only to composite materials (materials, components, etc.) but also to their manufacturing methods, etc. as appropriate. Even method-related components can be components related to products. Which embodiment is best depends on the target, required performance, etc.

[0021] <Heat-conducting particles (raw material)> (1) Form The thermally conductive particles may be in any form that allows them to be unevenly distributed between the islands of the resin. For example, when observing a cross section of the composite, the average minimum dimension of the thermally conductive particles should be smaller than the average minimum dimension of the spacing between the islands. The average minimum dimension can be determined, for example, as the arithmetic mean of the minimum dimensions of the target object in a randomly selected region of the cross section (e.g., 440 μm × 630 μm).

[0022] The arithmetic mean values ​​referred to in this specification may be those determined for all particles within a predetermined area, or may be those determined for 5 to 100 particles, 10 to 50 particles, or even 15 to 30 particles randomly sampled from within the predetermined area (the same applies hereinafter). Measurements and calculations based on observed images may be performed using image processing software (e.g., ImageJ) (the same applies hereinafter).

[0023] The thermally conductive particles should also have a shape that allows (and facilitates) crowding, aggregation, orientation, etc. between the island-shaped portions. For example, the thermally conductive particles should be flat (scale-like, plate-like, etc.) or fibrous. As an indicator of such a shape, the thermally conductive particles may have an average aspect ratio (AR) of 2 to 300, 5 to 100, or even 20 to 50. If the average aspect ratio is too small (e.g., approximately spherical), the gaps between the thermally conductive particles increase, which can reduce the packing, crowding, contact, orientation, etc., of the thermally conductive particles between the island-shaped portions. Although the average aspect ratio may be large, thermally conductive particles with an excessively large aspect ratio are difficult to handle.

[0024] The average aspect ratio is calculated, for example, as the arithmetic mean value of the aspect ratios of the objects in a predetermined area (e.g., 440 μm × 630 μm) of a randomly selected cross section (particularly the cross section of the interposed portion). The aspect ratio is the ratio of the longitudinal length measured for each object to the length in the direction perpendicular to that (longitudinal length / perpendicular length). The longitudinal length is the maximum length (maximum length of the line segment) of the object (thermal conductive particle).

[0025] The average aspect ratio can be calculated based on the measured values ​​of each thermally conductive particle in the composite, or it can be the average aspect ratio of the thermally conductive particles (single particles) at the raw material stage (before mixing with the resin raw material). For example, for flat thermally conductive particles (such as BN particles), the average aspect ratio (average particle size / average thickness) calculated based on their average thickness and average particle size can be used.

[0026] The thermally conductive particles (primary particles) should be smaller than the islands of the resin. For example, when observing a cross section of the composite, the average maximum dimension of the thermally conductive particles should be smaller than the average maximum dimension of the islands. The average maximum dimension can be determined, for example, as the arithmetic mean of the maximum dimensions of objects within a randomly selected area of ​​the cross section (e.g., 440 μm × 630 μm).

[0027] (2)Content ratio The thermally conductive particles are contained in the entire composite material in an amount of, for example, 5 to 70 volume %, 15 to 65 volume %, 30 to 60 volume %, or even 40 to 55 volume %. If the amount of thermally conductive particles is too small, the thermal conductivity of the composite material may decrease. On the other hand, the amount of thermally conductive particles increases, but an increase in the amount of expensive thermally conductive particles also increases the manufacturing cost of the composite material. According to the present invention, the thermal conductivity of the composite material can be improved while suppressing the content of thermally conductive particles. The volume fraction of the thermally conductive particles does not need to be measured directly; it can be calculated, for example, from the mass ratio (blending mass ratio) of the resin and the thermally conductive particles and their respective true densities.

[0028] (3) Type The type (material, manufacturing method, etc.) of the thermally conductive particles is not important as long as they satisfy the above-mentioned conditions. Examples of thermally conductive particles include silica (SiO2), alumina (Al2O3), and aluminum nitride (AlN). However, boron nitride (BN) is preferable because it has a higher thermal conductivity than silica or alumina and is more chemically stable than aluminum nitride. This results in a composite material (thermal conductive member) with excellent thermal conductivity, heat resistance, long-term reliability, electrical insulation, etc.

[0029] Boron nitride generally exists in a hexagonal normal pressure phase (also referred to as "h-BN") and a cubic high pressure phase (also referred to as "c-BN"). h-BN consists of scale-like (high aspect ratio flat) stacked hexagonal mesh layers similar to graphite, and has thermal conductivity anisotropy, with the thermal conductivity differing greatly between the plane direction (a-axis direction) and the thickness direction (c-axis direction).

[0030] The thermal conductivity of a composite can be improved by orienting the surface direction of thermally conductive particles (hereinafter simply referred to as "BN particles") made of h-BN in the primary thermal conduction direction (e.g., from the heat source side to the cooling source side). However, the more the orientation of the BN particles is biased (i.e., the more anisotropic), the lower the thermal conductivity of the composite in other directions, which can reduce the ease of handling and versatility of the composite. Therefore, it is more preferable for the orientation of the BN particles in the composite to vary to a certain extent. The filler of the composite may consist solely of BN particles, or it may contain other types of particles (ceramic particles, metal particles, c-BN particles, etc.).

[0031] The BN particles (primary particles) have an average particle size of, for example, 1 to 100 μm, 10 to 60 μm, 14 to 40 μm, or 18 to 30 μm. The average thickness is, for example, 0.01 to 5 μm, 0.15 to 4 μm, 0.3 to 3 μm, 0.5 to 2.5 μm, or 1 to 2 μm. BN particles of an appropriate size facilitate aggregation, crowding, or orientation between the island portions of the resin, which can contribute to improving the thermal conductivity of the composite material.

[0032] The particle size and thickness of BN particles in a composite are determined as the arithmetic mean of the dimensions of the target object within a predetermined area (e.g., 440 μm × 630 μm) of the cross section. For BN particles (thermal conductive particle raw material, usually powder) before mixing with the resin raw material, the average particle size may be determined as the 50% diameter (D50: median diameter) from the particle size distribution obtained by laser diffraction. The average thickness may also be determined as the arithmetic mean of the thicknesses measured for BN particles randomly sampled (e.g., 10–100 particles) from a microscopic image of the powder. If it is difficult to determine the size (particle size and thickness) of the filler contained in the composite based on an observation image (e.g., SEM image), the resin may be removed from the composite by dissolving or the like, and the extracted filler may then be subjected to laser diffraction or other methods to determine the filler size. The filler size determined in this manner is approximately the same as the filler size before mixing.

[0033] 《Resin (raw material)》 (1) The resin forms islands and holds (supports) the thermally conductive particles. The islands may occupy, for example, 30 to 80%, 35 to 70%, 40 to 60%, or even 45 to 55% of the total cross-sectional area of ​​the composite. The area ratio of the islands can be calculated, for example, by image processing a predetermined region (e.g., 440 μm × 630 μm) of the observed image (cross section).

[0034] When there are resin intervening portions between the island-shaped portions, the intervening portions may occupy, for example, 1 to 10%, 2 to 8%, 3 to 6%, or even 4 to 5% of the total cross-sectional area of ​​the composite material. The area ratio of the intervening portions can be calculated, for example, by image processing a predetermined region (e.g., 50 μm × 50 μm) between the island-shaped portions of the observed image (cross section).

[0035] The island-shaped portions are preferably larger than the thermally conductive particles (primary particles). For example, as described above, when observing a cross section of the composite, the average maximum dimension of the island-shaped portions is preferably larger than the average maximum dimension of the thermally conductive particles.

[0036] (2) The resin may be of a single type or multiple types. Using a single type of resin simplifies the raw material blending and manufacturing process. If multiple types of resin are used, it is possible to allocate suitable resins to the island portions and intervening portions, taking into account the characteristics of each resin (thermal conductivity, wettability with thermally conductive particles, softening point, melting point, etc.).

[0037] The resin (including rubber, elastomer, etc.) is appropriately selected depending on the specifications of the composite material (insulation, thermal conductivity, moldability, etc.). As the resin, a synthetic resin (even a polymer), which is a high molecular compound, is usually used. The resin may be a thermoplastic resin or a thermosetting resin. Examples of thermoplastic resins include polystyrene, acrylic resins such as polymethyl methacrylate (polymethyl methacrylate / PMMA), polycarbonate, polyphenylene sulfide, etc. Examples of thermosetting resins include epoxy resin, phenol resin, silicone resin, etc.

[0038] The elastomer may be, for example, an olefin-based (TPO), polystyrene-based (TPS), polyvinyl chloride-based (TPVC), polyurethane-based (TPU), polyester-based (TPC), polyamide-based (TPAE), etc. The rubber may be, for example, ethylene-propylene-diene rubber (EPDM), butyl rubber, or a thermosetting elastomer (urethane rubber, silicone rubber, fluororubber, etc.).

[0039] (3) It is desirable that the resin (resin raw material) at least in the raw material stage has a high elastic modulus. If the elastic modulus of the resin is too low, the amount of deformation of the resin during molding will be excessive, which may reduce the adhesion between the resin (especially the island portions) and the thermally conductive particles (and ultimately the thermal conductivity of the composite).

[0040] The elastic modulus (Young's modulus) of the resin is preferably, for example, 0.1 GPa or more, 0.5 GPa or more, 1 GPa or more, 1.5 GPa or more, or even 2 GPa or more. The upper limit of the elastic modulus of the resin is, if specified, for example, 15 GPa, 10 GPa, or even 5 GPa.

[0041] Because it is difficult to determine the elastic modulus of a resin during or after molding (composite material), the "elastic modulus" referred to in this specification refers to the elastic modulus of the resin raw material unless otherwise specified. It is sufficient to use the nominal value listed in the catalog for the resin used. If no such nominal value is available, the elastic modulus of the resin can be determined using a viscoelasticity spectrometer.

[0042] Examples of the Young's modulus of typical resins are PMMA: 2.5 GPa, olefin elastomer: <0.1 GPa, and phenol resin: 3 to 5 GPa.

[0043] 《Manufacturing method》 The composite material is obtained, for example, by molding a mixture of thermally conductive particle (filler) raw material and resin (matrix) raw material.

[0044] (1)Mixtures / raw materials The island-shaped portions may be formed during molding or during subsequent solidification. Formation of the island-shaped portions is facilitated when granular resin that can become the island-shaped portions is already contained in the mixture (e.g., resin raw material).

[0045] The granular resin may be in the form of raw resin particles, or a portion thereof (e.g., the surface layer) may be softened or melted. The mixture may also be in a solid-liquid coexistence state (semi-fluid state) in which granular resin and molten (including softened) resin coexist. The size and proportion (solid fraction) of the granular resin may be adjusted by controlling the heating temperature and heating time (heating step) taking into consideration the type of resin, softening point, melting point, etc. For example, the heating temperature may be 80 to 200°C, or 100 to 150°C, and the heating time may be 0.1 to 1 hour, or 0.3 to 0.7 hours.

[0046] (2) Molding process Molding the mixture under pressure can easily produce a composite material with high thermal conductivity, in which the thermally conductive particles are densely distributed among the resin islands. Molding can be performed by, for example, compression molding, extrusion molding, injection molding, transfer molding, etc.

[0047] The molding pressure (compression force) applied to the mixture is, for example, 10 to 500 MPa, 50 to 400 MPa, or even 100 to 300 MPa. If the molding pressure is too low, a dense (high thermal conductivity) composite material cannot be obtained, while if the molding pressure is too high, productivity may decrease and costs may increase.

[0048] The molding step may be performed in multiple stages. For example, the molding step may include a first molding step in which the mixture is pressure-molded without heating to obtain a molded body, and a second molding step in which the molded body is heated to a temperature equal to or higher than the softening point of the resin raw material and pressure-molded.

[0049] (3) Supplementary information The mixture may contain a coupling agent that enhances the affinity between the thermally conductive particles and the resin. The coupling agent may be introduced by surface treatment of the thermally conductive particles (coupling treatment, hydrophobic treatment, etc.), or may be added when mixing (kneading) the thermally conductive particles with the resin. Examples of the coupling agent include hexamethyldisilazane (HMDS: CH 19 There are silane coupling agents such as NSi2).

[0050] In addition to the thermally conductive particles, the composite may contain auxiliary particles with different materials, properties, particle sizes, etc. When the thermally conductive particles are h-BN particles, the auxiliary particles may be, for example, c-BN particles, carbon particles (nanocarbon particles, graphite particles, diamond particles, etc.), etc. The auxiliary particles may be smaller or larger than the island portions.

[0051] When the resin is a thermosetting resin, it is preferable to perform a heat hardening treatment (cure treatment) after molding. The composite material may be in the shape of the final product or a shape close to it, or it may be a material or intermediate material to be processed later.

[0052] <Anisotropy of thermal conductivity> The anisotropy of thermal conductivity depends on the degree of variation (degree of orientation) of the thermally conductive particles between the islands. For example, a composite that meets the following conditions can suppress the anisotropy of thermal conductivity (make it isotropic) and exhibit high thermal conductivity in directions other than a specific direction (one direction).

[0053] (1) Small area The composite may have a plurality of dispersed small regions between the island-shaped portions where the orientation of the thermally conductive particles differs considerably. For example, for each square-shaped small region with a side length twice the average particle size of the thermally conductive particles, the average inclination angle (referred to as the "first orientation degree") of the thermally conductive particles between the island-shaped portions with respect to the reference direction is determined. The first orientation degrees thus determined are compared, and it is preferable that there are at least two small regions where the difference in the first orientation degree is, for example, 30° or more, 40° or more, or even 50° or more. Such small regions can be extracted, for example, by selecting regions where the orientation of the thermally conductive particles differs based on an observation image of the regions between the island-shaped portions of the composite.

[0054] The average particle size of the thermally conductive particles can be determined using the method described above (such as laser diffraction). The first orientation degree can be calculated as follows: A small square region is extracted from the observation image, and a square grid (lattice) is drawn, dividing one side into five equal parts. The angle θ1 (0°≦θ1≦90° / θ1: absolute value) between the longest direction of the thermally conductive particles at or (very) close to each grid point (a total of 6 × 6 = 36 points) and one grid line (e.g., a vertical line / reference direction) is determined. The sum of these angles (Σθ1) divided by the number of measurements (N≦36) is defined as the first orientation degree (Σθ1 / N).

[0055] If there are no thermally conductive particles on or near a grid point, they are excluded from the number of measurements, and the actual number of measurements is set to N. If the vertical grid lines are taken as the reference direction, the angle between the horizontal grid lines is 90°. The reference direction (the direction in which the small regions are cut out and the grid lines are set) can be selected arbitrarily. However, thermally conductive particles with a large aspect ratio (such as BN particles) tend to have their longitudinal direction oriented in a direction approximately perpendicular to the pressure direction during molding (i.e., they tend to be oriented). For this reason, it is recommended to use the direction in which thermally conductive particles are likely to be oriented (the direction perpendicular to the pressure direction) as the reference direction.

[0056] Furthermore, the average value of the first orientation degrees calculated for the plurality of small regions may be, for example, 30° or more, 35° or more, or even 40° or more. If a plurality of small regions without bias in the orientation of the thermally conductive particles are extracted, the anisotropy of the thermal conductivity of the composite may be evaluated based solely on the average value of the first orientation degrees of the small regions, independently of the difference in the first orientation degrees described above. In this case, it is preferable to use the average value of the first orientation degrees calculated for at least three small regions.

[0057] (2) Large area In the composite, the thermally conductive particles between the island-like portions are preferably dispersed in various orientations. For example, the second orientation, which is the average value of the inclination angle of the thermally conductive particles between the island-like portions relative to the reference direction, measured in a large square region with sides five times the average particle size of the thermally conductive particles, is preferably 20° or more, 25° or more, or even 30° or more. Because the large region has an area more than six times that of the small region, the anisotropy of the thermal conductivity of the composite can be evaluated based on just the average value of the inclination angle (second orientation) in the large region.

[0058] The second degree of orientation is calculated in the same way as the first degree of orientation. That is, the angle θ2 (0°≦θ2≦90° / θ2: absolute value) between the grid line (reference direction) in one direction and the longest direction of the thermal conductive particles at each grid point (total of 6 × 6 = 36 points) drawn in the large area or in the vicinity thereof is calculated. The sum of these angles (Σθ2) divided by the number of measurements (N≦36) is the second degree of orientation (Σθ2 / N). Note that if there are no thermal conductive particles near the grid point (for example, when they overlap with an island-shaped portion), this amount can be subtracted from the number of measurements, as described above.

[0059] (3) Anisotropy of thermal conductivity The anisotropy of the thermal conductivity of a composite can be indexed, for example, by the ratio (λ1 / λ2) calculated from the thermal conductivities λ1 and λ2 (λ1 > λ2) measured in each orthogonal direction. It is desirable for this ratio to be, for example, 3 or less, 2.5 or less, or even 2 or less. The two orthogonal directions for measuring thermal conductivity may be set arbitrarily. Incidentally, similar to the reference direction described above, if the thermal conductivity measured in the direction in which thermally conductive particles are likely to be oriented (the direction orthogonal to the pressure application direction) is designated as λ1, the ratio (λ1 / λ2) can be the largest. Even in this case, it is desirable for λ1 / λ2 to be within the range described above.

[0060] 《Composite material》 The composite material is used, for example, in heat dissipation members, substrates, housings, etc. of electronic devices. Such a composite material preferably has high thermal conductivity and high specific resistance (electrical resistivity). The thermal conductivity of the composite material is, for example, 10 to 60 W / mK, 15 to 50 W / mK, or even 30 to 40 W / mK. A composite material with reduced anisotropy of thermal conductivity may be used by aligning the direction in which the thermal conductivity is relatively high with the main heat flux direction. [Example]

[0061] The present invention will be described in detail with reference to the following examples, in which several types of samples (composite materials) made of thermally conductive particles and resin were produced and the properties of each sample were evaluated.

[0062] [First Example] 《Raw materials》 (1) Thermally conductive particles Commercially available h-BN powder (manufactured by Denka Co., Ltd.) and AlN powder (manufactured by Showa Denko K.K.) were used as the thermally conductive particle source (raw material). Two types of h-BN powder with different particle morphologies (average aspect ratios) were prepared. The thermal conductivity (longitudinal / planar directions) of the BN particles was approximately 200 W / mK, and the thermal conductivity of the AlN particles was approximately 230 W / mK. The average aspect ratio of each particle will be described later.

[0063] (2) Resin As a resin raw material, granular (approximately spherical) polymethyl methacrylate (PMMA, simply referred to as "granular resin") was prepared as follows.

[0064] 2 g of polyvinyl alcohol (degree of polymerization: 2000) was added to 210 ml of water with stirring, and the temperature was raised to 90° C., after which it was dissolved over 1 hour.

[0065] The solution was cooled to room temperature and then transferred to a round-bottom flask equipped with a mechanical stirrer. 90 g of methyl methacrylate and 0.9 g of an oil-soluble azo polymerization initiator (AIBN) were added to the solution, and the mixture was stirred under a nitrogen stream (stirrer speed: 300 rpm) for 30 minutes. All raw materials used were reagents manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0066] The mixture was then stirred under a nitrogen stream and held at 60°C for 1 hour, followed by 70°C for 10 hours to polymerize the monomer (methyl methacrylate). The treated product was allowed to cool to room temperature, filtered through a screen mesh, washed with water and methanol, and then vacuum dried. The residue was sieved using a 500 μm mesh sieve, and 26 g of particles that passed through were collected. Microscopic observation revealed that each particle was roughly spherical with a diameter of approximately 200 to 500 μm. The elastic modulus of this granular resin was 2.5 GPa. The elastic modulus was measured using a viscoelasticity spectrometer (Iwamoto Seisakusho Co., Ltd.) as described above. The measurement conditions were: sample: film (thickness: 0.5 mm, width: 5 mm, grip length: 10 mm), frequency: 10 Hz, temperature range: Rt to 200°C, and program speed: 10°C / min.

[0067] <<Manufacturing of composite materials>> For each thermally conductive particle, a composite material in which the shape of the granular resin was partially retained (semi-molten) (referred to as an "unevenly distributed sample") and a composite material in which the shape had disappeared (referred to as a "uniform sample") were produced as follows.

[0068] (1) Unevenly distributed sample Each thermally conductive particle (filler) and the above-mentioned granular resin were weighed and mixed so that each thermally conductive particle accounted for 50% by volume of the entire composite material. The mixed thermally conductive particles and granular resin were then mixed manually.

[0069] The mixture was filled into a mold cavity (4 × 16 × 2 mm) and heated (mold temperature: 130°C for 30 minutes) (heating step). The heated mixture was cooled to room temperature while being uniaxially pressurized (300 MPa) in the mold (molding step). The solidified material (composite material) was then removed from the cavity and used as the unevenly distributed samples (samples 1 to 3).

[0070] (2) Uniform sample The completely melted liquid resin and thermally conductive particles were mixed by hand and then vacuum dried at room temperature for 24 hours. The mixture was then molded in a mold in the same way as the unevenly distributed sample to obtain a uniform sample.

[0071] Observation and Measurement (1) SEM Each sample was cut out, embedded in resin, polished, and the cross section was observed using a scanning electron microscope (SEM). The observed images are shown in Figures 1A to 1C (collectively referred to as "Figure 1"). Each enlarged photograph in Figure 1 shows the area where thermally conductive particles are present.

[0072] (2) Average Aspect Ratio (AR) Based on the SEM images of each uniform sample, the aspect ratio of the thermally conductive particles (maximum length in the longitudinal direction / length in the direction perpendicular to that) was measured and calculated, and the results are summarized in Figure 2. The average aspect ratio thus determined is shown in each figure as the AR value.

[0073] (3) Area ratio Image processing (software: ImageJ) was performed on the SEM images of each unevenly distributed sample, and the area ratio of each resin in the island and interstitial regions to the whole was calculated. Each area ratio is also shown in each figure.

[0074] The area ratio of the island-shaped portions was calculated based on the basic region (440 μm × 630 μm). The area ratio of the intervening portions was calculated based on an enlarged region (50 μm × 50 μm) of the remaining region after removing the island-shaped portions from the basic region. Specifically, the image was binarized using ImageJ, with the black region representing the intervening portion and the white region representing the filler, and the area ratio of the black region was used as the area ratio of the intervening portion.

[0075] (4) Thermal conductivity The thermal conductivity (λ) of each sample was determined using the Nanoflash method. Specifically, the thermal conductivity was calculated as λ = α·Cp·D using the thermal diffusivity (α) measured by the Nanoflash method (measuring device: NETZSCH LFA447), the specific heat (Cp) determined by a differential scanning calorimeter (DSC), and the density (D) calculated from the volume fraction of each material.

[0076] The thermal diffusivity of each sample was measured in a direction perpendicular to the direction of pressure applied during production. The thermal conductivity (thickness direction of the test piece) of each sample thus obtained is summarized in Figure 3.

[0077] [Second Example] (1) Sample preparation An unevenly distributed sample (sample 4) was also produced by replacing the PMMA used in Example 1 with an olefin-based elastomer. The olefin-based elastomer used was OE-90 manufactured by M-Tech Chemical Co., Ltd. The elastic modulus was measured in the same manner as in Example 1 and was found to be 0.1 GPa. A mixture of BN particles (AR=10) as thermally conductive particles and the olefin-based elastomer as a granular resin was heated at a mold temperature of 130°C for 30 minutes (heating step). The other steps were the same as in Example 1.

[0078] (2) Observation and measurement The sample thus obtained was also observed and measured in the same manner as in Example 1. The SEM image is shown in Figure 4A, and its thermal conductivity is shown in Figure 4B. Together, these two figures are simply referred to as "Figure 4."

[0079] [evaluation] As is clear from Figures 1, 3, and 4, all composites in which flat thermally conductive particles (BN particles) are unevenly distributed between the island-like portions of the resin exhibit high thermal conductivity (10 W / mK or higher) even when the thermally conductive particles are about 50% by volume. Furthermore, as is clear from a comparison of Samples 1 and 2 (Figure 3) with Sample 4 (Figure 4B), composites using resins with a high modulus of elasticity also exhibit higher thermal conductivity.

[0080] [Consideration] (1) Morphology of thermally conductive particles As is clear from the comparison of Samples 1 and 2 with Sample 3 shown in Figure 3, the thermal conductivity of the composite varied significantly depending on the morphology (average aspect ratio) of the thermally conductive particles, even among samples with uneven distribution. The reason for this is thought to be as follows: As shown in Figure 5, flat thermally conductive particles (AR >> 1) have a large area of ​​contact or proximity between adjacent particles. In this case, sufficient thermal conduction paths are easily formed between the thermally conductive particles. On the other hand, for roughly spherical thermally conductive particles (AR ≒ 1), the area of ​​contact or proximity between adjacent particles is very small, and there are many gaps where the resin is interposed. In this case, it is difficult for thermal conduction paths to be formed between the thermally conductive particles. Therefore, it is thought that the thermal conductivity varied significantly depending on the morphology (aspect ratio) of the thermally conductive particles, even among samples with uneven distribution.

[0081] (2) Island-like part As is clear from Sample 1 and Sample 2 in Figure 3, even when the same flat thermally conductive particles were used, the thermal conductivity of the composite varied significantly depending on whether it was an unevenly distributed sample or a uniformly distributed sample. The reason for this is thought to be as follows: Even when the proportion of thermally conductive particles in the composite as a whole was the same, the unevenly distributed sample with islands had closer thermally conductive particles (fillers) than the uniformly distributed sample without islands. Referring to the schematic diagram in Figure 6, where n is the number of fillers, f is the filler width, p is the island width, and d1 and d0 are the filler spacings, d1 = d0 - 2p / n. In other words, for the same area, the unevenly distributed sample with islands has fillers closer together by (2p / n) than the uniformly distributed sample without islands. The larger the island (p), the closer the fillers are to each other. Thus, even when the same flat thermally conductive particles were used, the presence or absence of islands is thought to have significantly different thermal conductivities.

[0082] (3) Type of resin As is clear from a comparison of Samples 1 and 2 shown in Fig. 3 with Sample 4 shown in Fig. 4B, even among the unevenly distributed samples, Sample 4 had a lower thermal conductivity than Samples 1 and 2. This is thought to be due to the difference in the thermal conductivity of the resin itself.

[0083] However, Sample 4 was more flexible than Samples 1 and 2. For this reason, it is believed that the composite material (sheet, etc.) of Sample 4 can be placed in close contact between a heat-generating component (electronic device, etc.) and a heat-dissipating component (heat sink, etc.), and can sufficiently reduce the thermal resistance between the two.

[0084] Therefore, in any case, it was confirmed that by using a composite material having island-shaped portions, high thermal conductivity can be achieved while suppressing the amount of thermally conductive particles.

[0085] [Third Example] 《Raw materials》 (1) Thermally conductive particles Three types of commercially available h-BN powder were prepared as the thermal conductive particle source (raw material). These were two types of agglomerated powders (average particle size: 150 μm and 90 μm, manufactured by Dandong Chemical Engineering Institute Co.) consisting of roughly spherical secondary particles, and a base powder (PT110, manufactured by Denka Co.) consisting of primary particles. The base powders were used after adjusting their average particle size to 18 μm and 10 μm. The particle size adjustment was performed by shearing, which cleaved the primary particles. Micrographs (SEM images) of these particles are shown in Figure 7. The average particle size of the primary particles obtained by crushing the secondary particles of the agglomerated powder was 11 μm.

[0086] (2) Resin Two types of granular resin were prepared as resin raw materials. One was a phenolic resin (LPS series manufactured by Lignite Corporation). The other was the PMMA mentioned above. When observed under a microscope, both were found to be roughly spherical with diameters of approximately 200 to 500 μm. The elastic modulus of the phenolic resin was measured using the method mentioned above and was found to be 3 GPa.

[0087] <<Manufacturing of composite materials>> The following sample (composite material) was manufactured by combining one type of these thermally conductive particles and one type of resin. The manufacturing method was almost the same as that for the unevenly distributed sample shown in Example 1. However, the heating process was performed at a mold temperature of 150°C for 10 minutes. Sample 21: Agglomerated powder (average particle size: 150 μm) + phenolic resin Sample 22: Agglomerated powder (average particle size: 90 μm) + phenolic resin Sample C1: Base powder (average particle size: 18 μm) + phenolic resin Sample C2: Base powder (average particle size: 10 μm) + phenolic resin Sample C3: Basic powder (average particle size: 18μm) +PMMA

[0088] Observation and Measurement (1) SEM Each sample was cut out, embedded in resin, polished, etc., and the cross section was observed with a scanning electron microscope (SEM). SEM images of samples 21, 22, and C1 are shown in Figure 8.

[0089] (2) Average Aspect Ratio (AR) Based on each SEM image, the aspect ratio of the thermally conductive particles (maximum length in the longitudinal direction / length perpendicular to that) was measured and calculated using the method described above. Samples 21 and 22 had an aspect ratio of 20, samples C1 and C3 had an aspect ratio of 25, and sample C2 had an aspect ratio of 20.

[0090] (3) Area ratio Based on each SEM image, the area ratio of the island and intervening resins to the total area was calculated using the method described above. For samples 21 and 22, the area ratio of the island and intervening resins was 48.58% and 49.46%, respectively. For sample C1, the area ratio of the island and intervening resins was 49% and 4.3%, respectively.

[0091] (4) Thermal conductivity The thermal conductivities (λ1, λ2) in two perpendicular directions were determined for each sample using the method described above. λ1 was the thermal conductivity in the direction perpendicular to the pressure direction. The thermal conductivity ratio (λ1 / λ2) for each sample is summarized in Figure 9.

[0092] (5) Degree of orientation For each of samples 21, 22, and C1, the orientation degrees of three small regions (first orientation degree) and one large region (second orientation degree) were determined. The small and large regions were cut out from between the island-like regions (intervening regions) of the composite material of each sample. The calculation process for each orientation degree is shown in Figures 10A-C (collectively referred to as "Figure 10") and 11A-C (collectively referred to as "Figure 11").

[0093] The small areas of samples 21 and 22 are square with sides measuring 22 μm (twice the average particle size of the decomposed h-BN particles, 11 μm). The large areas of samples 21 and 22 are square with sides measuring 55 μm (5 times the average particle size of 11 μm). The small areas of sample C1 are square with sides measuring 36 μm (twice the average particle size of 18 μm) of the h-BN particles. The large areas of sample C1 are square with sides measuring 90 μm (5 times the average particle size of 18 μm).

[0094] The angles of each h-BN particle on the grid points formed by dividing each side into five equal parts were measured, and the arithmetic mean value was calculated. The vertical lines of the grids shown in Figures 10 and 11 are perpendicular to the pressure direction (the measurement direction of λ1).

[0095] The first orientation degree (G1) and second orientation degree (G2) thus obtained are shown in Figures 10 and 11. In addition, for each sample, the difference (ΔG1) obtained by subtracting the minimum value from the maximum value among the three G1s, and the average value of the three G1s (G1m) are also shown in Figure 10.

[0096] "evaluation" As can be seen by comparing Figures 9, 10, and 11, sample C1, which has a large thermal conductivity anisotropy, has h-BN particles oriented in one direction (vertical), and ΔG1, G1m, and G2 are all relatively small. On the other hand, samples 21 and 22, which have a small thermal conductivity anisotropy, have h-BN particles oriented in various directions, and ΔG1, G1m, and G2 are all sufficiently large.

[0097] From the above, it was found that the anisotropy of thermal conductivity of composites with island-shaped portions can be suppressed. It was also confirmed that the anisotropy of thermal conductivity can be indexed by the degree of orientation.

Claims

1. A composite material made of a resin and thermally conductive particles having a higher thermal conductivity than the resin, the resin contains an acrylic resin or a phenolic resin and has granular island portions; the thermally conductive particles are flat or fibrous; A composite material in which, when a first orientation degree, which is the average value of the inclination angle of the thermal conductive particles between the island-shaped portions relative to a reference direction, is calculated for each square-shaped small region with one side twice the average particle size of the thermal conductive particles, there are multiple small regions in which the first orientation degree differs by 30° or more.

2. A composite material made of a resin and thermally conductive particles having a higher thermal conductivity than the resin, the resin contains an acrylic resin or a phenolic resin and has granular island portions; the thermally conductive particles are flat or fibrous; A composite material having a second orientation degree of 20° or more, which is the average value of the inclination angle of the thermal conductive particles between the island-shaped portions relative to a reference direction, measured in a large square area with one side five times the average particle size of the thermal conductive particles.

3. A composite material made of a resin and thermally conductive particles having a higher thermal conductivity than the resin, the resin contains an acrylic resin or a phenolic resin and has granular island portions; the thermally conductive particles are flat or fibrous; A composite material in which the ratio (λ1 / λ2) of thermal conductivities measured in two orthogonal directions (λ1≧λ2) is 3 or less.

4. 4. The composite material according to claim 1, wherein the thermally conductive particles include BN particles made of hexagonal boron nitride.

5. a molding step of pressurizing and molding a mixture of a resin raw material and a thermally conductive particle raw material; At least a portion of the resin raw material is granular, at least a part of the thermally conductive particle raw material is a secondary particle formed by agglomeration of flat or fibrous thermally conductive particles, A manufacturing method for obtaining the composite material according to any one of claims 1 to 4.

6. The method for producing a composite material according to claim 5, wherein the molding step comprises cooling the mixture heated to a softening point or higher of the resin raw material while applying pressure to the mixture.

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